This patent application is related to co-pending U.S. patent application. Ser. No. 08/153,724, filed on even date herewith, entitled "METHOD AND APPARATUS FOR MULTIPLE REPLY REJECTION WHEN DECODING TRANSPONDER REPLY SIGNALS"; and Ser. No. 08/153,722, also filed on even date herewith, entitled "METHOD AND APPARATUS FOR ASSOCIATING TARGET REPLIES WITH TARGET SIGNATURES"; both co-pending applications being commonly owned by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference.
The invention relates generally to detecting and decoding reply signals transmitted from aircraft transponders. More specifically, the invention relates to such detecting and decoding under conditions in which multiple reply signals are received during a receive interval following an interrogation.
Air traffic control and safety are ongoing concerns in commercial and military aviation. Particularly significant concerns are traffic alert and collision avoidance between aircraft either in route between or in the vicinity of landing fields. Ever increasing air traffic demands have resulted in governmental regulations that require commercial carriers to equip planes with active interrogation systems that can determine the presence and threat of nearby aircraft. The particular system mandated by the government depends on the aircraft size. Large commercial aircraft that carry over 30 passengers are being equipped with an active traffic alert and collision avoidance system (TCAS II) that not only detects and displays nearby aircraft, but also alerts the crew as to impending collisions, and also provides resolution advisories such as audible instructions to the pilot to pull up or down, maintain level or climb rate and so forth. This system, however, is very complex and expensive and therefore has not been mandated for smaller aircraft.
For aircraft that carry up to 30 passengers, governmental regulations require such aircraft be equipped with an active traffic alert and collision avoidance system (TCAS I) that detects nearby aircraft, determines and displays range, bearing and altitude of such aircraft relative to the interrogating plane, and tracks such aircraft within a prescribed range and issues an audible alert to the crew as to impending collisions. Although the operational performance of the TCAS I system appears less complex than TCAS II, numerous problems arise that make a cost effective system difficult to realize.
The Federal Aviation Administration (FAA) specifies that the TCAS I active interrogation systems use air traffic control radar beacon system (ATCRBS) signals. These ATCRBS interrogation signals are high frequency pulse amplitude modulated signals at 1030 Megahertz. The reply signals are also pulse amplitude modulated but at a carrier frequency of 1090 Megahertz. In TCAS I, the interrogation signals are transmitted from an interrogating aircraft to other aircraft in the vicinity thereof, and these other aircraft respond to the interrogations via a transponder located on the aircraft.
The interrogation and reply signal waveforms are specified by the FAA. The information contained in the reply signal depends on the type of interrogation (e.g. Mode A, Mode C) and the transponder equipment that the interrogated aircraft has available for responding. For TCAS I, the interrogation mode is Mode C, and the Mode C reply signal from the aircraft transponder consists primarily of encoded altitude data. The altitude data is encoded using binary logic states or bits arranged in four digit octal codes (i.e. each octal altitude code has twelve data bits with each octal digit defined by three data bits). The reply signal data bits are transmitted within a pair of framing pulses called bracket pulses that indicate (for purposes of TCAS I) the beginning and end of an altitude code reply signal from a particular aircraft responding to an interrogation.
A TCAS I system is specified based on the use of these ATCRBS Mode C reply signal waveforms. Thus, an interrogating aircraft may transmit an interrogation signal at 1030 MHz, and then will "listen" for Mode C reply signals from all aircraft capable of responding by transmitting the bracket pulses and altitude encoded data pulses. Some aircraft are not equipped to reply with altitude data (non-altitude reporting, or NAR) and hence only transmit the bracket pulses. Under TCAS I, aircraft within a range of about 34 nautical miles will reply to a Mode C interrogation.
When only a few aircrafts are within range for responding to a TCAS I interrogation, reply signal decoding is uncomplicated by the relative lack of interfering or overlapping replies. However, as the number of planes replying to an interrogation increases, reply signals received by an interrogating aircraft can overlap in time and thus produce what is referred to as synchronous garble. Synchronous garble basically refers to the presence of legitimate reply signals from different aircraft that are properly responding to the same interrogation such that their reply signals are overlapped in time when received by the interrogating plane. A TCAS compatible system must be able to separate the overlapping replies to a specified degree of accuracy by correctly associating received pulses to their respective reply signal.
Another problem in high density traffic areas arises from the presence of multiple ground based radars in the FAA secondary surveillance radar (SSR) system that can also be transmitting interrogations to aircraft, such as with Mode C and Mode A interrogations. Mode A and C replies to such SSR interrogations may be detected by any aircraft listening for Mode C replies, even though these replies are not in response to an aircraft interrogation. Also, multiple aircraft can be sending out Mode C interrogations. Aircrafts listening for replies will detect (if within range) the reply signals from nearby aircraft responding to these interrogations. Both these types of replies are asynchronous garble with respect to an interrogating aircraft, and are generally referred to as fruit replies. The fruit replies can overlap legitimate reply signals in a manner similar to the synchronous garble and must also be filtered.
Such garbled and overlapping reply signals make detection and decoding of the legitimate reply signals very difficult, particularly in areas with high density of transponder equipped aircraft where the TCAS systems are most needed. This is because prior systems typically were hardware intensive for detecting and decoding reply signals. Furthermore, even when reply signals could be detected by such systems, the presence of garbled signals could produce inaccurate decoded information.
The objective exists, therefore, for a traffic alert and collision avoidance system that utilizes and implements a reply signal decoding technique that can successfully discriminate overlapping reply signals and accurately decode garbled signals. Such a system also should be capable of identifying garbled information.